How to Make dusty effect on a surface?
There are a couple ways of doing this. Here's one way in cycles as your question didn't specify which engine you are using.


Start by adding an input>geometry node using the normal output.
Next, add a Separate XYZ node, you will be using the Z output.
A color ramp will be next, this will control the mix between the
dust layer and the rest of the material. Plug this and the preceding 2 nodes into the Fac of
the mix shader as shown below.
You can either use a diffuse shader colored as you want your dust, or
use a suitable image for your dust, it will go in the bottom input of
a mix shader.


Here is the node setup with an image texture, to use without, simply don't use it.


This is a suzanne with the above material: